Substitution effects on spin fluctuations in the spin-Peierls compound CuGeO_3

Using Raman scattering we studied the effect of substitutions on 1D spin fluctuations in CuGeO_3 observed as a spinon continuum in frustration induced exchange scattering. For temperatures below the spin-Peierls transition (T_{SP}=14K) the intensity of this continuum at 120-500 cm^{-1} is exponentially suppressed and transferred into a 3D two-magnon density of states. Besides a spin-Peierls gap-induced mode at 30 cm^{-1} and additional modes at 105 and 370 cm^{-1} are observed. Substitution of Zn on the Cu-site and Si on the Ge-site of CuGeO_3 quenches easily the spin-Peierls state. Consequently a suppression of the spin-Peierls gap observable below T_{SP}=14K as well as a change of the temperature dependence of the spinon continuum are observed. These effects are discussed in the context of a dimensional crossover of this compound below T_{SP} and strong spin-lattice interaction.Comment: 9 pages, 2 eps figures include

Magnetic Bound States in Dimerized Quantum Spin Systems

Magnetic bound states are a general phenomenon in low dimensional antiferromagnets with gapped singlet states. Using Raman scattering on three compounds as dedicated examples we show how exchange topology, dimensionality, defects and thermal fluctuations influence the properties and the spectral weight of these states.Comment: 3 pages, 1 figure, proceedings of the SCES'98, Paris, to be published in Physica

Effects of in-chain and off-chain substitutions on spin fluctuations in the spin-Peierls compound CuGeO_3

The effect of in-chain and off-chain substitutions on 1D spin fluctuations in the spin-Peierls compound CuGeO_3 has been studied using Raman scattering in order to understand the interplay between defect induced states, enhanced spin-spin correlations and the ground state of low dimensional systems. In-chain and off-chain substitutions quench the spin-Peierls state and induce 3D antiferromagnetic order at T\leq 5 K. Consequently a suppression of a 1D gap-induced mode as well as a constant intensity of a spinon continuum are observed at low temperatures. A 3D two-magnon density of states now gradually extends to higher temperatures T\leq 60K compared with pure CuGeO_3. This effect is more pronounced in the case of off-chain substitutions (Si) for which a N\'eel state occurs over a larger substitution range, starting at very low concentrations. Besides, additional low energy excitations are induced. These effects, i.e. the shift of a dimensional crossover to higher temperatures are due to an enhancement of the spin-spin correlations induced by a small amount of substitutions. The results are compared with recent Monte Carlo studies on substituted spin ladders, pointing to a similar instability of coupled, dimerized spin chains and spin ladders upon substitution.Comment: 14 pages, 6 eps figures, to be published in PR

Interference of a first-order transition with the formation of a spin-Peierls state in alpha'-NaV2O5?

We present results of high-resolution thermal-expansion and specific-heat measurements on single crystalline alpha'-NaV2O5. We find clear evidence for two almost degenerate phase transitions associated with the formation of the dimerized state around 33K: A sharp first-order transition at T1=(33+-0.1)K slightly below the onset of a second-order transition at T2onset around (34+-0.1)K. The latter is accompanied by pronounced spontaneous strains. Our results are consistent with a structural transformation at T1 induced by the incipient spin-Peierls (SP) order parameter above T2=TSP.Comment: 5 pages, 7 figure

Investigation of the Spin-Peierls transition in CuGeO_3 by Raman scattering

Raman experiments on the spin-Peierls compound CuGeO$_3$ and the substituted (Cu$_{1- x}$,Zn$_x$)GeO$_3$ and Cu(Ge$_{1-x}$,Ga$_x$)O$_3$ compounds were performed in order to investigate the response of specific magnetic excitations of the one-dimensional spin-1/2 chain to spin anisotropies and substitution-induced disorder. In pure CuGeO$_3$, in addition to normal phonon scattering which is not affected at all by the spin-Peierls transition, four types of magnetic scattering features were observed. Below T$_{SP}$=14 K a singlet-triplet excitation at 30 cm$^{-1}$, two-magnon scattering from 30 to 227 cm$^{-1}$ and folded phonon modes at 369 and 819 cm$^{-1}$ were identified. They were assigned by their temperature dependence and lineshape. For temperatures between the spin-Peierls transition T$_{SP}$ and approximately 100 K a broad intensity maximum centered at 300 cm$^{-1}$ is observed.Comment: 7 pages, LaTex2e, including 3 figures (eps) to be published in Physica B (1996

The J_1-J_2 model revisited : Phenomenology of CuGeO_3

We present a mean field solution of the antiferromagnetic Heisenberg chain with nearest (J_1) and next to nearest neighbor (J_2) interactions. This solution provides a way to estimate the effects of frustration. We calculate the temperature-dependent spin-wave velocity, v_s(T) and discuss the possibility to determine the magnitude of frustration J_2/J_1 present in quasi 1D compounds from measurements of v_s(T). We compute the thermodynamic susceptibility at finite temperatures and compare it with the observed susceptibility of the spin-Peierls compound CuGeO_3. We also use the method to study the two-magnon Raman continuum observed in CuGeO_3 above the spin-Peierls transition.Comment: Phys. Rev.

Infrared study of spin-Peierls compound alpha'-NaV2O5

Infrared reflectance of alpha'-NaV2O5 single crystals in the frequency range from 50 cm-1 to 10000 cm-1 was studied for a, b and c-polarisations. In addition to phonon modes identification, for the a-polarised spectrum a broad continuum absorption in the range of 1D magnetic excitation energies was found. The strong near-IR absorption band at 0.8 eV shows a strong anisotropy with vanishing intensity in c-polarisation. Activation of new phonons due to the lattice dimerisation were detected below 35K as well as pretransitional structural fluctuations up to 65K.Comment: 3 pages, 2 figures, 1 table. Contributed paper for the SCES'98 (15-18 July 1998, Paris). To be published in Physica

Magnetic bound states in the quarter-filled ladder system α′−NaV2O5\alpha'-NaV_{2}O_{5}}

Raman scattering in the quarter-filled spin ladder system alpha'-NaV_2O_5 shows in the dimerized singlet ground state ($T \leq T_{SP}=35K$) an unexpected sequence of three magnetic bound states. Our results suggest that the recently proposed mapping onto an effective spin chain for $T > T_{SP}$ has to be given up in favor of the full topology and exchange paths of a ladder in the dimerized phase for $T < T_{SP}$. As the new ground state we propose a dynamic superposition of energetically nearly degenerate dimer configurations on the ladder.Comment: 5 pages, 4 figures, to be published in PRB, brief reports, Dec. 199

Effectiveness of lurasidone in schizophrenia or schizoaffective patients switched from other antipsychotics: a 6-month, open-label, extension study

Objective. To evaluate the long-term safety and tolerability of lurasidone in schizophrenia and schizoaffective disorder patients switched to lurasidone. Method. Patients in this multicenter, 6-month open-label, flexible-dose, extension study had completed a core 6-week randomized trial in which clinically stable, but symptomatic, outpatients with schizophrenia or schizoaffective disorder were switched to lurasidone. Patients started the extension study on treatment with the same dose of lurasidone taken at study endpoint of the 6-week core study; following this, lurasidone was flexibly dosed (40-120 mg/day), if clinically indicated, starting on Day 7 of the extension study. The primary safety endpoints were the proportion of patients with treatment emergent adverse events (AEs), serious AEs, or who discontinued due to AEs. Secondary endpoints included metabolic variables and measures of extrapyramidal symptoms and akathisia, as well as the Positive and Negative Syndrome Scale (PANSS), Clinical Global Impressions-Severity (CGI-S), and the Calgary Depression Scale for Schizophrenia (CDSS). The study was conducted from August 2010 to November 2011. Results. Of the 198 patients who completed the 6-week core study, 149 (75.3%) entered the extension study and 148 received study medication. A total of 98 patients (65.8%) completed the 6-month extension study. Lurasidone 40, 80, and 120 mg were the modal daily doses for 19 (12.8%), 65 (43.9%), and 64 (43.2%) of patients, respectively. Overall mean (SD) daily lurasidone dose was 102.0 mg (77.1). The most commonly reported AEs were insomnia (13 patients [8.8%]), nausea (13 patients [8.8%]), akathisia (12 patients [8.1%]), and anxiety (9 patients [6.1%]). A total of 16 patients (10.8%) had at least one AE leading to discontinuation from the study. Consistent with prior studies of lurasidone, there was no signal for clinically relevant adverse changes in body weight, lipids, glucose, insulin, or prolactin. Movement disorder rating scales did not demonstrate meaningful changes. Treatment failure (defined as any occurrence of discontinuation due to insufficient clinical response, exacerbation of underlying disease, or AE) was observed for 19 patients (12.8% of patients entering) and median time to treatment failure was 58 days (95% CI 22-86). The discontinuation rate due to any cause was 50/148 (33.8%), and median time to discontinuation was 62 days (95% CI 30-75). The mean PANSS total score, mean CGI-S score, and mean CDSS score decreased consistently from core study baseline across extension visits, indicating an improvement in overall condition. Conclusions. In this 6-month, open-label extension study, treatment with lurasidone was generally well-tolerated with sustained improvement in efficacy measures observed in outpatients with schizophrenia or schizoaffective disorder who had switched to lurasidone from a broad range of antipsychotic agents